In 1500 he had convinced his brother and co-duke to conquer Dithmarschen, and a great army was called from not only the duchies, but with additions from all of the Kalmar Union for which his brother briefly was king. In addition, numerous German mercenaries took part. The expedition failed miserably, however, in the Battle of Hemmingstedt, where one third of all knights of Schleswig and Holstein lost their lives.[3]

A group of Jutish nobles had offered Frederick the throne as early as 1513, when his brother, King John, died, but he had declined, rightly believing that the majority of the Danish nobility would be loyal to prince Christian.

In 1523 his nephew Christian II, the King of Denmark, Norway and Sweden, was forced by disloyal nobles to abdicate, and Frederick took the throne as King Frederick I. It is not certain that Frederick ever learned to speak Danish. After becoming king, he continued spending most of his time at Gottorp, a castle and estate in the city of Schleswig.

In 1524 and 1525 Frederick had to suppress revolts among the peasants in Jutland and Scania who demanded the restoration of Christian II. The high point of the rebellion came in 1525 when Søren Norby, the governor (statholder) of Gotland, invaded Blekinge in an attempt to restore Christian II to power. He raised 8000 men who besieged Kärnan (Helsingborgs slott), a castle in Helsingborg. Frederick's general, Johann Rantzau, moved his army to Scania and defeated the peasants soundly in April and May 1525. After the Battle at Lund, rebels fled into the cathedral and Rantzau's soldiers dragged 60 men from the church and executed them on the spot. Approximately 3,000 rebels died before the uprising ended.[citation needed]

Gold coin or medal of Frederick I. Shows him together with Sophia on the obverse, and coat of arms on the reverse.

Frederick played a central role in the spread of Lutheran teaching throughout Denmark. In his coronation charter he was made the solemn protector (værner) of the Catholic Church in Denmark. In that role, he asserted his right to select bishops for the Catholic dioceses in the country. Christian II had been intolerant of Protestant teaching, but Frederick took a more opportunist approach. For example, he ordered that Lutherans and Catholics were to share the same churches. He encouraged publication of the first Danish language Bible. When Hans Tausen was threatened with arrest and trial for heresy, Frederick appointed him his personal chaplain to give him immunity in 1526. Starting in 1527, Frederick authorized the closure of Franciscan houses and monasteries in 28 Danish cities.[4] In some cases, he offered small sums of money to the displaced monks.[4] He used the popular anti-establishment feelings that ran against some persons of the Catholic hierarchy and the Catholic nobility of Denmark as well as keen propaganda to decrease the power of bishops and Catholic nobles. He was skillful enough to prevent all-out warfare between Protestants and Catholics.

In 1532 he succeeded in capturing Christian II who had tried to make a political come-back in Norway. Tensions between Catholics and Protestants rose to a fever pitch which resulted in the Count's Feud (Grevens Fejde) upon Frederick's death. As King of Norway, Frederick is most remarkable in never having visited the country. He was never crowned King of Norway, and therefore styled himself King of Denmark, the Vends and the Goths, elected King of Norway. Frederick died on 10 April 1533 in Gottorp, at the age of 61, and was buried in Schleswig Cathedral.[5]

When still a Duke, Fredrick's first wife was Catharina died in child birth in 1491, leaving them with a daughter, who he named after his wife who passed - Kathrina, who went on to marry General Hermann Hoyer, son of Margaret Von Schack.